There is certainly a great deal of evidence in W. W. Jacob's short story "The Monkey's Paw" to show the reader that both Mr. and Mrs. White have come to believe Sergeant-Major Morris's legend about the monkey's paw.
The first point of evidence concerns the fact that, when Morris throws the paw into the burning fire in the fireplace, Mr. White immediately snatches the paw off of the fire, saying, "If you don't want it, Morris ... give it to me." A second point of evidence concerns the fact that Mr. White next asks Morris how to make a wish on the paw. We see further evidence when Mr. White tells both his son Herbert and his wife that he offered Morris payment for the paw. Mr. White would not have done any of these things had he believed the legend was nothing more than just a tall tale. From these facts, it's clear that seeing his friend's earnestness in saying his wishes were granted and that someone he knew died on account of wishing for death significantly influenced him into accepting the story.
Though both Herbert and Mrs. White show a lot of doubt in the beginning of the story, Mrs. White comes to believe in the legend once Mr. White is paid £200 for the death of their son. The most significant piece of evidence showing her belief concerns the fact that, at bedtime, she suddenly asks her husband if he has destroyed the paw and where it is. She then informs him she has just thought of wishing "our boy alive again." She would not have thought of doing this if she didn't believe at this point there was truth in the legend.